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REFRACTORIES                                           509
deliberate attempt is made to arrive at the composition A12O3 SiO,
f ™?t0rieS Hke the M^quardt porcelain used for pyrometer tubes consist 8lUrre "^ SUffiClent feMspar t0 brin* abo^a dense Btoicture at V 7i rr a miXtoe W°Uld C°nsist °f a Calcine fired to cone 20 having the compos.t.on 71.66 per cent kaolin and 28.34 of anhydrous alumina bonded together by clay. The mixture then would be as follows:
PER
CENT
Sillimanitc (calcined).......... 55
Ball clay (raw)............... 12
Kaolin (raw)................. 13
PER CENT
Alumina (calcined)...........  10
Feldspar......................  10
When necessary the plasticity of the mixture can be improved through the use of more clay or increase of the total content of raw clay.
Magnesite Refractories.—The mineral magnesite, MgC03, is the source of basic refractories used as furnace linings for various metallurgical processes, in the form of bricks or in the crushed state as a mortar and cement for furnace bottoms. The moHt prominent uses of these materials are in the basic open-hearth furnace and electric-furnace linings.
Both crystalline and crypto-crystalline magnesite are used in this connection, the former having a specific gravity of 3.02, the latter of 2.9-3.0. The most important Hourees of this mineral are Austria-Hungary (Styria, Lower Austria and Northern Hungary), Greece, Canada (Quebec), Washington and California. The latter two Hourees represent chiefly the crypto-crystalline variety.
The chemical coin position of the several types of magnesite is given in the following table:
TABLE 7
	Styria	Quebec	Washington	California
	(calcined),	(calcined),	(calcined),	(crude),
	per cent	per cent	per cent	per cent
Silica,  .          ....   ........	2.8	5.5	6.8	3.86
Ferric, oxide ............	7.1	7.0	5.0	0.80
Alumina        .                 ....	0.0	2.0	2.0	
Magnesium oxide ..........	Hf> . (>	67.5	83.1	43.47
Liine	2.9	17.7	3.1	2.00
( <nrbon dioxide, ............				49.48
In the manufacture of these refractories it is necessary to calcine the magnesite before it can be molded owing to the large shrinkage which the mineral undergoes and it is desirable to reduce it, as much as possible, to a condition where it will remain fairly constant, in volume upon refining. Owing to the exceedingly high temperatures